1. Technical Field
This invention relates generally to dielectric heating systems, and more particularly to solid state radio frequency (RF) generators for dielectric heating of food products.
2. Description of the Background Art
Dielectric heating applies an intense, rapidly alternating electric field to materials to be heated. Electrically asymmetric or polar molecules in the materials to be heated constantly attempt to move and rotate to align themselves with the changing electric field. The electrical agitation of these polar molecules produces heat.
Water absorbs electrical energy in this manner particularly well, so dielectric heaters are often used where preferential heating of water is needed. Examples of these uses include drying paper, wood, ceramics, ink, textiles, and foam. Dielectric heating can also be used for distillation, curing glues, sintering, soil remediation, medical waste sterilization, and freeze-drying.
Dielectric heaters are also used where other means of heating are difficult to implement or have undesirable consequences. For example, contact heating involves application of heat to a surface of a material and then waiting for the heat to conduct to other portions of the material. This is inefficient for materials that do not conduct heat well and materials which cannot withstand very high contact temperatures, as is the case with most plastics and solid fruit pieces.
Ohmic heating involves passing an electrical current directly through a conductive material to be heated. Without stirring, ohmic heating does not necessarily uniformily heat all portions of the material and can overheat localized volumes. Sterilization and cleaning are also difficult because ohmic heating requires a contacting electrode.
Induction heating applies an intense, rapidly alternating magnetic field, to induce an electrical current to flow and cause heating in a material. Induction heating, however, does not work particularly well on non-metallic materials.
Microwave heating has become more popular in recent years, but also has limitations. Microwaves do not necessarily penetrate deeply into materials to be heated and, due to standing waves in the heater, do not penetrate uniformly, again possibly causing localized overheating and undesired flavor degradation in fruit pieces.
Dielectric heating is substantially instantaneous and does not require direct contact between a hot surface and the material to be heated. There is no conventional ohmic current passed through the material, so a material to be heated does not need to be electrically conductive, only capacitively lossy. Unlike the induction heating case, dielectrically heated materials do not need to be metallic. Dielectric heaters operating at radio frequencies, often tens of MHz, produce substantially uniform heating, and thus avoid localized overheating. Radio frequency dielectric heaters do not need complex waveguides or shielding measures, and do not use inefficient magnetron energy sources as do most microwave heaters. Further, generation of significant radio frequency power output is usually more economical than production of an equivalently effective microwave power output.
Dielectric heating is particularly useful in the food processing industry. Foods can be baked, dried, and pasteurized with dielectric heating. Moisture content can be controlled very precisely because water is preferentially heated in dielectric heating systems. When the water evaporates, the electrical energy is no longer efficiently coupled into the food being heated, so heating is self-limiting. Widely used glass and TEFLON.RTM. containers are not capacitively lossy at radio frequencies, so RF dielectric heaters are easily adaptable to existing large-scale bulk food processing operations.
Conventional dielectric heaters use vacuum tubes to produce the kilowatts of RF energy needed for commercially useful equipment. Vacuum tubes are fragile, bulky, and failure-prone due to limited operational lifetime. High-power vacuum tubes are very expensive, on the order of $20,000 for a 100 kilowatt tube. These tubes are usually operated in inefficient Class A circuits, and often require elaborate water cooling systems that are cumbersome to maintain. Failure of one of these high-power vacuum tubes usually requires the entire heating system to be shut down for maintenance. A more affordable, reliable, and efficient means for generating RF power for dielectric heating of food products is needed.